1. Field of the Invention
This invention relates generally to rotary compressors and, more particularly, to a rotary compressor which is designed as a sealed casing type for use in air conditoners, and which has improved durability in a high speed operation.
2. Description of the Prior Art
The prior art related to the present invention will be described below with reference to FIGS. 7 to 14.
In a rotary compressor shown in FIG. 7, an electric motor 1 and a compression mechanism 2 are housed in a sealed casing 14 and connected together by a crankshaft 10.
The electric motor 1 is housed in an upper portion of the internal space of the sealed casing 14. The electric motor 1 has a rotor la and a stator 1b. The crankshaft 10 is fitted into and fixed to the rotor 1a to drive the compression mechanism 2.
The compression mechanism 2 has a cylinder block 3 fixed to the sealed casing 14, a rolling piston 4 rotatably mounted on an eccentric portion 10a of the crankshaft 10 positioned in a cylinder bore 3a formed in the cylinder block 3, a vane 5 reciprocally movable following the revolution of the rolling piston 4, main and sub bearings 6 and 7 which close the cylinder bore 3a at the upper and lower ends thereof, respectively, and which support the crankshaft 10, a discharge valve 9 provided on the sub bearing 7, and a cover for the discharge valve 9.
As balancing weights (hereinafter referred to as "balancers") for cancelling the force produced by the eccentric rotation of the rolling piston 4, balancers 11-1, 11-2, and 11-3 are provided. The balancer 11-1 is attached to the end portion of the crankshaft 10 adjacent to the sub bearing 7, and the second and third balancers 11-2 and 11-3 are attached to the lower end and the upper end of the rotor 1a, respectively. A cover 12 is provided for the first balancer 11-1.
FIG. 8 is an enlarged cross-sectional view of the compression mechanism 2.
A longitudinal hole 21 is formed in the crankshaft 10 to enable lubrication oil 13 reservoired at the bottom of the internal space 14a of the sealed casing 14 to be supplied to the sliding members through an oiling piece 20. Small lateral holes 22-1, 22-2, and 22-3 are also formed in the crankshaft 10 so as to communicate with the longitudinal hole 21, thereby enabling the oil to be supplied to oil grooves 23-2 and 23-1 respectively formed in the inner circumferential surfaces of the main and sub bearings 6 and 7.
In the structure of the conventional compression mechanism, as shown in FIG. 9 which is a cross-sectional view taken along the line IX--IX in FIG. 8, a gap 26 is formed between a shaft hole 25 of the sub bearing 7 and a thrust surface 24 of the crankshaft 10 because the thrust surface 24 is eccentric with respect to the shaft hole 25.
This type of structure is disclosed in Japanese Utility Model Unexamined Publication No. 59-107984.
In the above-described prior art, the changes caused by the rotation of the first balancer 11-1 in the pressures in spaces 26-1 and 26-2 defined between the rolling piston 4, the eccentric portion 10a of the crankshaft 10 and the sub and main bearings 7 and 6 were not taken into consideration. The conventional mechanism will be described below in more detail with respect to this point.
FIGS. 11A to 11D show the movement of the gap 26 shown in FIG. 9 caused by the rotation of the crankshaft 10. Hatchings in FIGS. 11A to 11D indicate the thrust surface 24.
In the position shown in FIG. 11A, the thrust surface 24 does not cover the oil groove 23-1 at all. In the position shown in FIG. 11A, the crankshaft 10 is rotated clockwise to cause the thrust surface 24 to start covering the oil groove 23-1. In the position shown in FIG. 11C, the thrust surface 24 completely covers the oil groove 23-1. In the position shown in FIG. 11D, the thrust bearing 24 starts opening the oil groove 23-1.
FIG. 12 shows changes in the area (opening) of the oil groove 23-1 not covered with the thrust surface 24 during one revolution of the crankshaft 10. As can be understood from FIG. 12, the oil groove 23-1 is opened and closed one time during one revolution of the crankshaft 10 and this cycle is repeated.
FIG. 13A shows changes in the pressure in the space 26-1 shown in FIG. 8 during one revolution of the crankshaft 10 in a low speed operation, while FIG. 13B shows changes in the pressure in the same space during one crankshaft revolution in a high speed operation. In the low speed operation, as shown in FIG. 13A, the pressure variation .DELTA.P.sub.Q1 is comparatively small. In the high speed operation, as shown in FIG. 13B, the pressure variation .DELTA.P.sub.Q2 is substantially large.
The reason will be explained below with reference to FIG. 10. As the crankshaft 10 rotates, the first balancer 11-1 acts as a centrifugal pumping to move the lubrication oil 13 in the cover 12 in the directions of arrows A. By this movement, the lubrication oil in the spacer 26-1 is drawn downward through the oil groove 23-1 by the first balancer 11-1 as indicated by arrows B in FIG. 10 when the thrust surface 24 opens the oil groove 23-1 as shown in FIG. 11A and 11B. The lubrication oil in the spacer 26-1 is not drawn by the first balancer 11-1 when the thrust surface 24 closes the oil groove 23-1 as shown in FIG. 11C and 11D. Consequently, each time when the oil groove 23-a is opened by the thrust surface 24, the lubrication oil flows in the space 26-1. As the rotational speed of the crankshaft 10 is increased to a speed of about 8000 rpm or higher, the drawing force of the first balancer 11-1 abruptly increases. Accordingly, the pressure variation .DELTA.P.sub.Q2 increases relative to the pressure variation .DELTA.P.sub.Q1 exhibited during low speed operation.
As the pressure in the space 26-1 is repeatedly greatly changed alternating stresses are caused in the members which define the space 26-1, resulting in the occurrence of a phenomenon i.e., cavitation abrasions in which the materials of the members are partially removed. A surface 28 of the sub bearing 7 facing the space 26-1 was damaged by cavitation abrasion. This surface 28 was observed with an electron microscope. Striation patterns peculiar to fatigue failure were found in the inner surface of a recess formed by the removal of the material. The occurrence of cavitation abrasion was also observed in a surface 29 of the main bearing 6 which faces the space 26-2.
The main bearing 6 is formed of graphite flake cast iron while the sub bearing 7 is formed of sintered iron alloy. The occurrence of cavitation abrasion in the surface of the main bearing 6 indicates that the variation in the pressure in the space 26-2 on the side of the main bearing 6 is substantially large. It is thought that the variation in the pressure in the space 26-1 adjacent to the sub bearing 7 influences the pressure in the space 26-2 adjacent to the main bearing 6 through the oil groove 23-3.
As described above, the conventional compressor structure is deigned without any consideration of pressure variations caused by the operation and therefore entails a problem in terms of durability during a high speed operation.